The present invention relates to a releasable locking device and more particularly to such a device for use in assembling end fittings to fuel assemblies for nuclear reactors.
Fuel assemblies forming the nuclear core of a nuclear reactor are generally comprised of a plurality of elongated fuel rods containing fuel or fissionable material which are grouped and joined together. Recently nuclear reactor fuel assemblies have been made wherein the structural support is provided by vertically extending hollow tubes which serve as guides for control rod elements and which are secured to upper and lower end plates. The fuel rods are aligned and supported within the structural framework formed by the guide tubes and end plates by spacer grids which are welded to the guide tubes and provide both lateral and, to some extent, axial restraint to the fuel rods.
Recently, it has been found desirable, from both the standpoint of neutron economy and dollar economy, to fabricate fuel assemblies using dissimilar metals. For example, U.S. Pat. No. 3,770,583 for a Fuel Assembly Hold Down Device in R. H. Klumb et al and copending application Ser. No. 373,207 for Fuel Assembly For A Nuclear Reactor by Anthony et al filed June 25, 1973 disclose a fuel assembly arrangement fabricated from the zirconium alloy Zircaloy for those components within the active core region and a material such as stainless steel or the nickel, chromium, iron alloy having the trade name Inconel for the remaining elements. Those components considered to be in the active core or fuel region include the vertically extending control rod guide tubes, spacer grids, the fuel rods and the cladding of the fuel rods themselves, while those components outside the active region include the upper and lower end plates or fittins and the means associated with the plates for supporting and maintaining the alignment of the fuel assembly within the reactor core.
Also it is known that it is desirable from the standpoint of cost and reactor efficiency, to have fuel assemblies in which defective fuel rods may be replaced or special fuel or burnable poison rods added after the assembly has been irradiated. Such versatility may preclude an entire assembly from being prematurely scrapped because of a relatively few defective fuel rods.
Copending U.S. patent application Ser. No. 373,207 filed June 25, 1973 and entitled "Fuel Assembly For A Nuclear Reactor" by A. J. Anthony et al, discloses an arrangement for a fuel assembly which incorporates both of the desirable features of being fabricated from two or more dissimilar materials and possessing the capability of being easily disassembled to provide access to the fuel rods for replacement thereof. As disclosed in that application, the upper and lower end fittings are attached to the fuel assembly so that they may be easily removed by means of threaded joints between the hollow structural members (i. e., guide tubes) and the means for alignment which extend from the end fittings for aligning and supporting the assembly within the reactor core.
It is to a locking device for these threaded joints to insure against retrograde movement of the alignment posts during normal reactor operation which the present invention is directed in one aspect thereof.